The present invention relates to a method for operating an SCR catalytic converter system which has a first SCR catalytic converter and a second SCR catalytic converter, to a computer program, to a machine-readable storage medium and to an electronic control device.
In order to satisfy ever stricter emissions legislation (Euro6, Tier2Bin5 and further-reaching emissions regulations), it is necessary to reduce nitrogen oxides (NOx) in the exhaust gas of internal combustion engines, in particular diesel engines. A known practice for this purpose is to arrange an SCR catalytic converter (selective catalytic reduction) in the exhaust region of internal combustion engines, said catalytic converter reducing nitrogen oxides contained in the exhaust gas of the internal combustion engine to nitrogen in the presence of a reducing agent. This enables the proportion of nitrogen oxides in the exhaust gas to be considerably reduced. The process of reduction requires ammonia (NH3), which is added to the exhaust gas. NH3 or reagents which release NH3 are therefore metered into the exhaust line. In general, an aqueous urea solution (urea/water solution) is used for this purpose, said solution being injected ahead of the SCR catalytic converter in the exhaust line. Ammonia forms from this solution and acts as a reducing agent. A 32.5% aqueous urea solution is commercially available under the trade name AdBlue®. In order to achieve high conversion rates of the nitrogen oxides to be reduced in an SCR catalytic converter system, the SCR catalytic converter must be operated in such a way that it is continuously filled with the reducing agent ammonia up to a certain level. The efficiency of an SCR catalytic converter is dependent on the temperature, on the space velocity and, very decisively also, on the NH3 filling level thereof.
SCR catalytic converters store a certain quantity of ammonia on their surface through absorption. In addition to the directly metered ammonia (in the form of urea/water solution), therefore, there is also stored NH3 available for NOx reduction, as a result of which efficiency is increased over a depleted catalytic converter. The storage behavior is dependent on the respective operating temperature of the catalytic converter. The lower the temperature, the higher is the storage capacity. If the reservoir of the catalytic converter is completely filled, however, there can be what is referred to as ammonia slip in the event of step changes in load, even if no more reducing agent is being metered in. In the case of ammonia slip, only some of the ammonia contained in the reducing agent and introduced into the exhaust gas upstream of the SCR catalytic converter is converted by means of the SCR catalytic converter.
If as high as possible NOx conversion rates are to be achieved, it is indispensable to operate the SCR system at a high NH3 filling level. In order to increase SCR efficiency more quickly after a cold start with a neutral impact on CO2 emissions, the SCR catalytic converter is installed closer to the engine and, in some cases, combined with a diesel particle filter (DPF) to form an “SCRF” catalytic converter (SCR catalytic converter on diesel particle filter). In such close proximity to the engine, however, the temperature gradients are also higher, and the temperature level rises to an absolute temperature level which is too high for SCR operation in the full load range. For this reason, a second SCR catalytic converter is therefore generally used, this converter optionally being mounted under the floor. To operate an SCRF/SCR system, particularly when two metering valves are used, an optimum interplay between the SCRF catalytic converter and the second SCR catalytic converter is required for optimum operation with very high NOx efficiencies. When using two metering valves, the first metering valve is mounted ahead of the SCRF catalytic converter and the second metering valve is mounted ahead of the second SCR catalytic converter.
In modern systems, there are two setpoint filling levels for an SCRF catalytic converter, a minimum level with a reduced NOx efficiency without or only with a relatively high NH3 slip and a maximum filling level for high NOx conversion with a low NH3 slip, up to about 200 ppm. First of all, the SCRF catalytic converter is operated with a maximum filling level, the NOx efficiency is very high, and the NH3 slip which occurs is absorbed by the second SCR catalytic converter. At a low NOx slip but a high NH3 slip from the SCRF catalytic converter, the NH3 filling level in the second SCR catalytic converter rises quickly beyond the minimum filling level of the second SCR catalytic converter. Even the minimum filling level in the second SCR catalytic converter causes high NOx conversion but there is still filling level capacity for NH3 slip from the SCRF catalytic converter. If the NH3 filling level in the second SCR catalytic converter is above the minimum filling level and below the maximum filling level, the setpoint NH3 filling level in the SCRF catalytic converter is lowered in accordance with an interpolation factor. If the filling level in the second SCR catalytic level rises as far as the maximum filling level or above, the setpoint NH3 filling level in the SCRF catalytic converter is accordingly lowered to a minimum filling level, ensuring that no NH3 slip occurs. The method described functions well and, in principle, a system having two SCR catalytic converters can be brought under control by this means. However, the method has the disadvantage that the response is only to the NH3 slip, i.e. to the NH3 filling level in the second SCR catalytic converter, not to a deviation of the current NH3 slip from the desired NH3 slip. This means that the control system can rapidly trend towards overshooting and that NOx performance may be lost in the process.